This invention relates to corrosion control in aromatic extraction units, and more particularly, to a corrosion inhibitor for glycol solvents utilized to selectively extract aromatics from aromatic-aliphatic mixtures.
Glycol is a well-known solvent for preferentially extracting aromatic compounds from a gas or liquid mixture of aromatic and aliphatic compounds. In a typical use, a gas or liquid stream containing aromatic compounds such as benzene, xylene, and toluene and aliphatic components such as propane, butane, pentane, and hexane is brought into intimate contact in an extraction zone with a glycol-water solvent solution containing more than about 90% by weight glycol under conditions such that the aromatic components are selectively extracted into the solvent. A glycol-aromatics extract and a raffinate phase consisting essentially of aliphatic compounds are formed and then separated in a suitable liquid-liquid or gas-liquid separator. The glycol-aromatics extract is directed to a distillation column wherein the desired aromatics are removed by azeotropic distillation from the glycol and recovered as a product. The glycol solvent is also recovered from the distillation column and, being essentially free of aromatics, is recycled as a fresh solvent feed to the extraction zone.
One problem inherent in operating a glycol-aromatic extraction unit is that of corrosion. Oxygen is known to enter the glycol solvent and cause corrosion both by direct reaction with iron in the steel equipment handling the glycol and by reaction with the glycol itself to produce a number of corrosive organic acids. The usual method utilized to combat such corrosion involves the addition of an organic amine, such as ethanolamine, to the glycol solvent, the ethanolamine functioning as a corrosion inhibitor and as an acid neutralizing agent. Although such amines reduce corrosion, it has been found that problems still remain. In particular, it has been found that corrosion still occurs when ethanolamine is utilized as a corrosion inhibitor in locations where the metal surface in contact with the glycol is at an elevated temperature, for example, above about 250.degree. F. Additionally, the corrosion products produced in the glycol solvent have been found to create further problems in that they plug small diameter piping leading to sensitive temperature and pressure control devices, causing operating problems, especially with respect to the operation of the distillation column separating the glycol from the aromatics.
Another problem involved in extracting aromatics with glycol is emulsion formation. Many additives which aid in controlling corrosion in glycol solutions prove undesirable because they tend to promote the formation of emulsions, which substantially lower the extractive efficiency of the glycol solvent, i.e., the emulsions reduce the capability of the glycol for extracting aromatics and thus lower the amount of aromatics produced on a daily basis from an extraction unit operating with a fixed amount of solvent recirculating at a given rate. The presence of emulsions in the glycol solvent often lowers the extractive efficiency of the solvent (and therefore the amount of products produced) by as much as 10 to 20 percent, sometimes even more.
It is an object of the invention, therefore, to provide an additive for glycol solvents utilized to extract aromatics from aromatic-aliphatic mixtures that substantially reduces corrosion without promoting emulsion formation. More specifically, it is an object of the present invention to substantially eliminate corrosion in glycol-aromatic extraction units without affecting the extractive efficiency of the glycol solvent. These and other objects will become apparent in view of the following description of the invention.